1. Field
Embodiments of the present invention relate to a method of making a semiconductor transistor.
2. Discussion of Related Art
In general, the basic process used in fabricating integrated circuits includes a material deposition stage, a patterning stage, a material removal stage, a doping stage, and a heating stage. The particular stages used depend on the type of devices to be included on the integrated circuit. For example, when the integrated circuit is silicon based with millions of transistors interconnected, the process takes into consideration that features of the transistor, such as the source, drain, and gate, must be formed and that the semiconductor material is silicon.
For such an integrated circuit, fabrication of the transistors typically involves growing a gate dielectric layer on a silicon substrate and forming a polysilicon gate electrode on the gate dielectric layer. Sometimes, shallow trench isolation (STI) regions are formed around the substrate to separate wells in one portion of the substrate from wells in another portion of the substrate. There may also be vertical sidewall spacers formed on the sides of the gate electrode and gate dielectric layer. The silicon substrate, STI regions, spacers, gate electrode, and gate dielectric layer form part of the transistor.
Nickel is deposited over the surface of the transistor and annealed to form NiSi regions in areas where the nickel is in contact with silicon and polysilicon. Thus NiSi is formed on the surface of the gate electrode and on the surface of the substrate, but un-reacted Ni remains on the STI regions and the spacers. The un-reacted Ni is removed by dipping the transistor in a solution of sulfuric acid (H2SO4), peroxide (H2O2), and water
(H2O) at a temperature around eighty degrees centigrade (80xc2x0 C. or 176xc2x0 Fahrenheit), which dissolves the un-reacted Ni the STI regions and the spacers and leaves NiSi to form the source, drain, and gate. This material removal stage is called xe2x80x9cwet etch.xe2x80x9d
The above process (sometimes called a xe2x80x9csalicidexe2x80x9d process) is adequate for making silicon-based semiconductor transistors, which are the majority of semiconductor transistors being made today. However, with new or other materials being contemplated, such a process may not be appropriate. This is because although fairly common, the particular wet etch chemicals at such temperatures will dissolve certain other semiconductor materials leaving no source, gate, or drain for the transistor.